FIG. 4 shows a vertical sectional front view of a dead tank type vacuum circuit breaker of earlier technology. A ground tank 2 is supported on a mount platform 1. An operation box 4 is fixed to one end of the ground tank 2 through a support plate 3. Operation box 4 includes therein an operation mechanism. An insulating support tube 5 is supported by the support plate 3 at one end in the horizontal direction in ground tank 2, and a support insulating member 6 is supported at the other end in the horizontal direction in ground tank 2. An electrically conductive movable-side contact case 8 is supported on the insulating support tube 5 through an insulating support member 7. A fixed-side contact case 9 is supported on the support insulating member 6. A vacuum interrupter 10 serving as a circuit breaking portion is supported horizontally at a movable-side end portion of the vacuum interrupter and a fixed-side end portion of the vacuum interrupter 10, respectively, by the movable-side and fixed-side contact cases 8 and 9. The operating mechanism in operation box 4 is connected with a movable lead 11 of vacuum interrupter 10 through a lever not shown in the figure and an insulating operating rod 12 extending through the insulating support tube 5 and the insulating support member 7. The movable lead 11 of vacuum interrupter 10 is inserted in the movable-side contact case 8, and electrically connected with the movable-side contact case 8. A fixed lead 13 of vacuum interrupter 10 is electrically connected with fixed-side contact box 9. Conductors 14 and 15 include lower ends electrically connected, respectively, with the contact cases 8 and 9, and extend upward in an inclined state, from the inside of ground tank 2. Conductors 14 and 15 are surrounded, respectively, by bushings 16 and 17, which are supported, respectively, by bushing current transformers 18 and 19 mounted on the ground tank 2. Bushing terminals 20 and 21 are provided, respectively, at upper ends of the conductors 14 and 15.
Furthermore, SF6 gas of about 0.15 MPa is filled in the ground tank 2 in order to insulate the high voltage main circuit section of conductors 14 and 15 and vacuum interrupter 10, and the ground tank 2 at a ground potential or earth potential. Since the SF6 gas is superior in insulating properties, the SF6 gas can perform its function at a low pressure.
FIG. 5 shows a sectional view of the vacuum interrupter 10 of the earlier technology. A vacuum vessel is formed by hermetically closing both ends of a ceramic insulating tube 22 with a metallic fixed-side end plate 23 and a movable-side end plate 24. One end of the fixed lead 13 is fixed to the center of the fixed-side end plate 23. The movable lead 11 extends through a through hole 24a formed at the center of the movable-side end plate 24. One end of a bellows 25 is fixed to the inner side of the movable-side end plate 24 around the through hole 24a. The other end of bellows 25 is fixed to the movable lead 11. Fixed electrode 26 and movable electrode 27 are fixed, respectively, to inner ends of fixed lead 13 and movable lead 11 so that the fixed electrode 26 and movable electrode 27 confront each other. A main shield 28 is provided on the inner side of insulating tube 22 at the middle in the length of insulating tube 22. Terminal shields 29 and 30 are provided on the inner sides of end plates 23 and 24, respectively. A bellows shield 31 is fixed to the movable lead 11 so as to cover a part of bellows 25.
In the thus-constructed vacuum circuit breaker, closing and opening operations are performed in the following manner. When the operating mechanism is driven in response to a closing command in the case of the closing operation, the movable lead 11 is moved through the lever and the insulating operating rod 12, and the movable lead 11 brings the movable electrode 27 into contact with fixed electrode 26, and thereby makes connection between conductors 14 and 15. When, in the case of the opening operation, the insulating operating rod 12 is pulled by the operating mechanism through the lever in response to an extracting command, the movable lead 11 is moved, and the movable lead 11 separates the movable electrode 27 from fixed electrode 26, and thereby breaks the connection between conductors 14 and 15.
In the vacuum interrupter 10, the bellows 25 capable of expanding and contracting maintains the vacuum in the vacuum vessel notwithstanding movement of the movable lead 11 in the closing and opening operations. Bellows 25 has a structure capable of bearing a pressure difference to some extent between the vacuum on the outer side and the pressure of the SF6 gas on the inner side. However, when the pressure difference increases beyond a certain level, the bellows 25 may suffer phenomenon called buckling since bellow 25 is made of thin sheet of metallic material such as stainless steel. Accordingly, the pressure of the SF6 gas on the inner side of bellows 25 needs to be lower than or equal to about 0.2 MPa. Moreover, for prevention of the global warming, it is required recently to reduce the quantity of usage of the SF6 gas as much as possible because of its higher global warming potential.
A patent document 1 shows a technique to prevent damage of a bellows by decreasing the difference between the inner and outer pressures of the bellows by the setting of a vacuum on the outer side of the bellow and a low pressure gas or an atmospheric pressure on the inner side of the bellows. A patent document 2 shows a technique of making a space on an anti-vacuum side a sealed gastight chamber of a low pressure.    Patent Document 1: Published Japanese Patent Application, Kokai No. 2004-220922    Patent Document 2: Published Japanese Patent Application, Kokai No. H06-208820